The present invention concerns a device associated with a flexible riser for a floating structure such as a drilling or production ship for recovery of oil and gas, where the riser extends from a wellhead on the seabed to a pipe system on the floating structure.
Since oil and gas are recovered from increasingly greater depths, there has been a growing need to use floating structures for such recovery. Since a floating structure moves in relation to the seabed, it is necessary to use flexible risers which, as mentioned above, extend from a wellhead on the seabed to a pipe system on the floating structure. The floating structure may be either dynamically positioned, or moored to the seabed by means of mooring lines. To enable the floating structure to turn in response to wind or weather, the riser and any relevant mooring lines are led via a so-called turret which is rotatably mounted on the structure. The flexible risers are normally led through a continuous guide pipe in the turret and are connected directly to the pipe system on the deck of the turret by means of couplings. The guide pipes are installed vertically in the turret, and to avoid the risers being subject to kinking and rubbing against the underside of the turret, it is commonly known either to supply the risers with bend restrictors or to supply the guide pipes with funnel-shaped ends, thereby to attempt to ensure that the flexible risers have a limited bending radius.
Using continuous guide piping makes it difficult or (depending on the diameter) impossible to inspect the part of the risers which is inside the guide pipes. One major disadvantage of using bend restrictors is that the flexible pipes are subject to extra strain because of the pressure forces exerted by the restrictors. These forces create pressure forces between the various layers within the flexible pipes, which in turn leads to internal wear and tear in the pipe walls. Also, wobble may occur between the attached restrictor and the pipe wall because of the internal wear of the pipe wall, wear between the pipe wall and the restrictor, and shrinkage of the plastic materials. This wobble may lead to the restrictor being displaced so that the riser is subject to harmful bending, and further external wear on the riser in the area where it enters the guide pipe. Detection of this wear and wobble cannot be made by external inspection. A disadvantage of using a funnel-shaped termination or end on the guide pipe is that the riser is subject to concentrated lateral loads. A further disadvantage is that wear occurs on the surface between the riser and the funnel, and internal wear occurs in the riser wall as a result of these concentrated lateral loads. To reduce wear on the riser, spacers are sometimes used between the riser and the guide pipe. However, these spacers make it quite impossible to inspect the part of the flexible riser which is inside the guide pipe.
Both the solutions described above, using a bend restrictor or a funnel-shaped part, thus entail a number of disadvantages which could lead to rupture of the riser and thus harmful release of oil and gas.